Automatic drum brake adjuster locked at a high temperature

ABSTRACT

A brake adjuster for adjusting the working clearance of the shoes of an automotive drum brake, the adjuster including three co-axial elements a first rod element ( 25 ) having a head portion for engagement with a first brake shoe and a threaded stem portion, an intermediate element ( 28 ) in threaded engagement with the first rod element and a second rod element ( 27 ) having a head portion for engagement with a second brake shoe and a stem portion slideably connected with the intermediate element ( 28 ) and free to rotate relative thereto. A reaction member ( 30 ) is mounted on the intermediate element ( 28 ) and a ring gear ( 35 ) is provided on the intermediate element which is axially located between the second rod element and the reaction member. A leaf spring ( 31 ) acts between the second rod element ( 27 ) and the reaction member ( 30 ) for pushing the second rod element away from the intermediate element ( 28 ). This leaf spring carries a bi-metallic pawl ( 31 ) which is either formed integrally with the leaf spring or is secured around the spring so that during a normal operation the pawl engages the ring gear ( 35 ) to operate the adjuster and at temperatures above a pre-set value the pawl is deflected out of engagement with the ring gear to prevent adjustment.

This invention relates to brake adjusters which automatically adjust theworking clearance of a brake shoe in automotive drum brake.

It is well known that brake drum expansion may lead to excessiveadjustment of brake clearances when the brake is hot and that when thebrake cools all the brake shoe clearance may disappear and the brakeshoe may become locked against the associated drum.

European patent EP-B-0538909 claims a brake adjuster which is fittedwith a temperature sensitive device which prevents adjustment of theassociated brake at high temperatures. The temperature sensitive devicedescribed in EP-B-0538909 is relatively complex and difficult tomanufacture.

It is an object of this invention to provide an improved form ofautomatic adjuster which is less expensive to produce.

Thus according to a first aspect of the present invention there isprovided a brake adjuster for adjusting the working clearance of theshoes of an automotive drum brake, the adjuster including three co-axialelements:

a first rod element having a head portion for engagement with a firstbrake shoe and a threaded stem portion;

an intermediate element in threaded engagement with the first rodelement;

a second rod element having a head portion for engagement with a secondbrake shoe and a stem portion slideably connected with the intermediateelement and free to rotate relative thereto;

a reaction member mounted on the intermediate element;

a ring gear for rotation with the intermediate element and axiallylocated between the second rod element and the reaction member;

a leaf spring acting between the second rod element and the reactionmember for pushing the second rod element away from the intermediateelement, said leaf spring carrying a pawl designed to engage the ringgear, and

a temperature sensitive mechanism designed to prevent the ring gearrotating when the brake temperature exceeds a pre-set value;

the adjuster being characterised in that the temperature sensitivemechanism is provided by forming either the leaf spring as a bimetallicmember or the pawl as a bimetallic member formed integrally on thespring or secured around the spring so that during a normal operationthe pawl engages the ring gear to operate the adjuster and attemperatures above a pre-set value the pawl is deflected out ofengagement with the ring gear to prevent adjustment.

Various alternative constructions are possible. For example, the leafspring may be formed from a first metal and the pawl which projects fromthe leaf spring as an integral part thereof may also be formed from thefirst metal and from a second metal laminated onto the first metal toprovide the bi-metal function.

Alternatively the leaf spring may be formed as a bi-metal lamination offirst and second metals and the pawl which is integral with the springand projects therefrom may be formed from the first metal only. In suchan arrangement the deflection of the pawl out of engagement with thering gear occurs as a result of changing curvature of the leaf spring.

In a further alternative both the leaf spring and the integral pawl mayboth be formed from a bi-metal laminate of first and second metals withboth the leaf spring and pawl being deflected as a result of changes intemperature.

In a further form of the invention, when the pawl is formed from abi-metal laminate which is secured around the spring, the pawl may haveformations which engage co-operating locating formations on the springsuch as flaps on the pawl which may be folded around the edges of thespring and which may engage recesses formed in the edges of the spring.

The present invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 is a side view of a vehicle drum brake including a brake adjusterembodying the present invention;

FIG. 2 is a perspective view of the brake adjuster used in FIG. 1;

FIG. 3 is a perspective view of a spring and pawl used in the adjusterof FIG. 2;

FIGS. 4 to 6 show a side view, plan view and a section on line L—L of afurther form of spring and pawl for use in the adjuster of FIG. 2.

Referring to FIG. 1 this shows a drum brake 10 having a pair of brakeshoes 11,12 which at their lower ends react against a fixed abutment 13and at their upper ends are forced apart by an hydraulic wheel cylinder14. The shoes are kept in contact with the abutment 13 by a lower returnspring 15 and in contact with the wheel cylinder 14 by an upper returnspring 16.

Thus during normal service operation the shoes are brought into contactwith the inner surface 17 of brake drum 18 in the conventional manner byactuating wheel cylinder 14 whose pistons 19 and 20 respectivelyoutwardly displace the contacting ends of shoes 11 and 12 to engage thedrum. In the conventional manner the two shoes are held down by holddown clips 21 and 22 respectively. The drum brake is also provided witha handbrake operating lever 23 pivoted at 24 on brake shoe 12. The handbrake operating lever contacts the right hand bead portion 25 of anadjuster strut 26 whose left hand head portion 27 contacts the otherbrake shoe 11. In conventional manner if, for example, the handbrakelever 23 is pivoted on pivot 24 in a clockwise sense as viewed in FIG. 1the handbrake lever will push the right hand head portion 25 of adjusterstrut 26 to the left thus pushing the brake shoe 11 to the left. This ineffect causes the two brake shoes 11 and 12 to move apart relative toeach other and hence again engage the inner surface 17 of the brake drum18 thus applying the hand brake.

The adjuster strut 26 basically comprises three components. The righthand head portion 25, the left hand head portion 27 and an intermediateelement 28. The right hand head portion 25 has a threaded stem portion29 which is received in a screw threaded bore in the right hand end ofintermediate element 28. Similarly the left hand head portion 27 has astem portion which is axially slideable within an internal bore in theleft hand bend of intermediate element 28. A sheet metal reaction member30 surrounds the intermediate element 28 and is axially located relativethereto. A bow spring 31 reacts at one end on an abutment 32 provided onthe reaction member 30 and at its other end on an abutment 33 providedon a clip like reaction member 34 mounted on the stem portion of lefthand head portion 27. A gear wheel 35 formed on intermediate element 28is engaged by a pawl 36 which forms part of bow spring 31.

Bow spring 31 biases left hand head portion 27 to the left relative tothe intermediate element 28. The effect of this is that when the lefthand end portion 27 is moved into the bore in the intermediate memberwhen the brake shoes are retracted by the springs 15 and 16 this causesthe bow spring 31 to assume a more bowed configuration so that the pawl36 tends to be drawn up out of engagement with the gear wheel tooth 35which is it is currently engaging. When the amount of movement of theleft hand head portion relative to the right hand portion exceeds apredetermined level (indicating a predetermined amount of retraction ofthe brake shoes which in turn indicates a predetermined amount of wear)then the pawl will engage the next tooth and gear wheel 35 and when thebrake is next applied the energy stored in the bowed spring rotates theintermediate element 28 slightly relative to the right hand head portion25 to increase the effective length of the adjuster strut 26 to take upthe wear.

Thus far described the adjuster strut is of known type.

In accordance with the present invention a temperature sensitivemechanism is provided to ensure disengagement of the pawl 36 from thegear wall 35 at high temperatures to prevent over adjustment of the drumbrake.

This temperature sensitive mechanism may, for example, take the form ofa bi-metal pawl 36 as shown in FIG. 3 in which the pawl comprises afirst metal layer 40 which is the same metal as the remainder of the bowspring and a second metal layer 41 which extends only over the pawlelement of the spring. With this type of construction when the bowspring heats up as a result of the general hot condition of the brakethe pawl will tend to deflect to the left as shown by arrow X thustending to disengage the gear wheel 35 and prevent over adjustment ofthe adjuster strut.

In an alternative construction both the bow spring 31 and the pawl 36are of a two layer bi-metal construction so that both elements deflecton reaching a particular temperature range thus disengaging the pawlfrom the gear wheel 35. A further variant is possible in which only thebow spring 35 is of a two layer bi-metal construction and the pawl 36 isa single metal layer. Again in this construction the heating up of thebow spring will cause sufficient deflection of the bi-metal bow springto disengage the pawl 36 from the gear wheel 35.

A yet still further alternative construction is shown in FIGS. 4 to 6 inwhich a separate bi-metal pawl member 50 is clipped around the edges ofthe bow spring 31. One edge of the bow spring has a projection 31 a andthe other edge of the bow spring has a recess 31 b. Two prongs 51straddle the projection 31 a and are bent round the edge of the bowspring 31. The other side of the bow spring to further prongs 52 arebent into the ends of recess 31 b thus retaining the pawl 50 mounted onthe intermediate portion of the bow spring 31 in a fixed axial position.In accordance with the invention the pawl 50 is formed from a two layerbi-metal element so that the pawl will deflect to the right as shown byarrow Y in FIG. 6 if the brake overheats and will thus disengage thegear wheel 35.

The present invention therefore provides a brake adjuster which includesa bi-metal temperature sensitive mechanism for disengaging the pawl fromthe gear wheel if the adjuster becomes too hot thus preventing overadjustment of the associated drum brake. The bi-metal temperaturesensitive mechanism constructions disclosed do not result in anysignificant impairing of the bow spring itself which can occur if stressconcentrations are induced by, for example, riveting the pawl to the bowspring.

1. A brake adjuster for adjusting the working clearance of the shoes ofan automotive drum brake, the adjuster including three co-axialelements: a first rod element having a head portion for engagement witha first brake shoe and a threaded stem portion; an intermediate elementin threaded engagement with the first rod element; a second rod elementhaving a head portion for engagement with a second brake shoe and a stemportion slideably connected with the intermediate element and free torotate relative thereto; a reaction member mounted on the intermediateelement; a ring gear for rotation with the intermediate element andaxially located between the second rod element and the reaction member;a leaf spring acting between the second rod element and the reactionmember for pushing the second rod element away from the intermediateelement, said leaf spring carrying a pawl designed to engage the ringgear, and a temperature sensitive mechanism designed to prevent the ringgear rotating when the brake temperature exceeds a pre-set value; theadjuster being characterised in that the temperature sensitive mechanismis provided by forming the pawl as a bimetallic laminate having flapsthereon which are folded around the edge of the spring and which engagerecesses in the edge of the spring so that during a normal operation thepawl engages the ring gear to operate the adjuster and at temperaturesabove a pre-set value the pawl is deflected out of engagement with thering gear to prevent adjustment.